This invention relates to a method of keeping the toner density of a developer constant in a developing device for an electrophotographic image forming machine such as a copier and more particularly to a method of setting a reference value for toner density when toner is added to into such a developer.
Many electrophotographic image forming devices such as copiers use a two-component developer composed of toner and carrier. The toner density in such a developer decreases gradually as the copying process is repeated and the toner becomes attached to transfer paper and is carried away. In order to keep the toner density of the developer constant all the time, there may be provided a detector adapted to detect a certain physical characteristic of the toner to thereby determine the toner density of the developer. The proper amount of toner to be freshly supplied can thus be determined. Copiers equipped with such a detector or a sensor have been disclosed, for example, in U.S. Pat. Nos. 3,892,672, 4,364,659 and 4,592,645 and in U.S. patent application Ser. No. 819,629 filed Jan. 17, 1986 and assigned to the present assignee.
With a detector or a sensor of this type, a reference value must be initially determined with respect to which the sensor output is compared because the sensor characteristics are generally not uniform, depending partially upon the circuit characteristics of the senor and the positions within a developer tank where the sensor is affixed and also because there are usually fluctuations among the individual sensors. One method of determining such a reference value for toner density would be to pour into the developer tank a two-component developer with a known toner concentration and, after the mixture is stirred until the sensor output is sufficiently stable, to set this output level as the toner concentration reference value. This method, however, is time-consuming and not very accurate because the reference voltage for the operational amplifier used for comparing the output signal from the toner density sensor with a reference level signal is adjusted manually.